This invention relates generally to covered rolls for industrial applications, and more particularly to rolls with relatively hard covers.
Covered rolls are used in demanding industrial environments where they are subjected to high dynamic loads and temperatures. For example, in a typical paper mill, large numbers of rolls are used not only for transporting the web sheet which becomes paper, but also for processing the web itself into finished paper. These rolls are precision elements of the system which should be precisely balanced with surfaces that are maintained at specific configurations.
One type of roll that is subjected to particularly high dynamic loads is a calendar roll. Calendaring is employed to improve the smoothness, gloss, printability and thickness of the paper. The calendaring section of a paper machine is a section where the rolls themselves contribute to the manufacturing or processing of the paper rather than merely transporting the web through the machine.
In order to function properly, calendar rolls generally have extremely hard surfaces. For example, typically calendar rolls are covered with a thermoset resin having a Shore D hardness between 84-95 and an elastic modules between 1,000-10,000 MPa. Most commonly, epoxy resins are used to cover calendar rolls because epoxy resins form extremely hard surfaces. Epoxy resins with characteristics suitable for forming the surfaces of calendar rolls are cured at relatively high temperatures (in the range of 100-150xc2x0 C.).
It is well known that an increase in curing temperature for heat resistant thermoset resin systems typically indicates an increased thermal resistance of the resulting cover. Present day demands of paper mills require rolls, particularly calendar rolls, with higher thermal resistances. Thus, it is desirable to produce covers for such rolls which can be cured at 150-200xc2x0 C.
However, curing at such high temperatures can cause so much residual stress within the cover that it tends to crack, rendering it unusable. A discussion of the physical chemistry of such a roll cover can be found in a paper entitled, xe2x80x9cThe Role Of Composite Roll Covers In Soft And Super Calendaring,xe2x80x9d J. A. Paasonen, presented at the 46xc3xa8me Congres Annuel Atip, Grenoble Atria World Trade Center Europole, Oct. 20-22, 1993, the teachings of which are incorporated herein by reference. Indeed, one important challenge to the manufacture of roll covers is to develop roll covers that can withstand the high residual stresses induced during manufacturing. Problems from residual stresses are most significant in harder compounds and often result in cracking, delamination, and edge lifting. In addition, residual stresses often cause premature local failure or shorter than desired life cycles. This is especially true for high performance, hard polymeric roll coverings, for which the basic approach has been to tolerate a production level of residual stresses that is still acceptable for product performance. Therefore, there is a need to develop methods of roll cover construction that reduce residual stresses in the product.
Consideration of residual stresses is especially critical during the manufacture of the roll cover. In particular, heating and curing processes must be given careful consideration, as these conditions are often the most significant factors in the development of such stresses. Residual stresses most often develop in polymer based covers as a result of the mismatch in thermal shrinkage properties between and/or among the cover materials and the core materials and from chemical shrinkage. Polymers typically have a coefficient of thermal expansion that is an order of magnitude greater than that of steel, the typical material of the core.
One suggestion to alleviate stresses caused by processing covered rolls is to produce a cover as a finished product and bond the fully cured cover to a core structure. This can be accomplished by wrapping a cover (topstock) over a mold, then demolding and bonding the cover to a core structure at a lower temperature level than the cover cure temperature, or by casting the cover separately and bonding it to a metal core at a lower temperature than the casting temperature.. Under these processes, the thermal stresses that would arise between the cover and the core from cooling the cover should be reduced.
Unfortunately, although adhesives for bonding the cover to the core are available, some adhesives exhibit poor bonding strengths when the roll is subjected to industrial applications. In general, adhesives that are suitable for high temperature performance also cure at high temperatures. Thus, subjecting the core to high temperature bonding conditions can result in stresses that were avoided by separately producing the cover.
In addition, manufacturing costs would be increased by producing the cover first as a separate cylindrical structure, then fitting it over a roll core at a lower processing temperature than was required for processing the cover. These casting methods require that an open cavity be created between the cover and the roll core, which necessitates multiple process steps and the use of inner mandrels. Even if the cover is separately manufactured via a centrifugal casting method, additional costs and steps are required for an outer mold.
Another possible solution is to develop a cover material having a thermal shrinkage as close to the metallic core as possible. While composite structures may be developed with the expansion coefficients tailored to match the metal core, such methods are expensive and may not produce the desired thermomechanical response for certain industrial applications. Thus, the need exists to develop methods to reduce the residual stress levels in current production materials.
In view of the foregoing, it is an object of this invention to reduce the problems caused by chemical and thermal shrinkage that develop during the manufacture of a covered roll.
The problems caused by chemical and thermal shrinkage of hard roll covers are reduced in accordance with the present invention by separately casting the cover with the inclusion of at least one intermediate compressive layer over a disposable inner mold. The inner mold is formed of a material that is rigid enough to support the cover during processing, and easily removed and discarded after processing. The intermediate layer which is applied over the mold is compressible enough to deform and absorb the stresses which develop as the cover is shrinking during processing.
The problems caused by chemical and thermal shrinkage are further reduced in accordance with the present invention through a method comprising the steps of applying the intermediate compressive layer over a disposable inner mold, applying a polymeric cover material over the intermediate compressive layer, and curing the cover material into a cylindrical cover at an elevated temperature. Next, the cover is permitted to shrink during curing or hardening, and the disposable inner mold is disposed of. The roll is completed by applying the cylindrical cover over a roll core base to form an intermediate roll having a circumferential gap layer, sealing both ends of the intermediate roll, and filling the gap layer with a filler material.
In another embodiment of the present invention, a metal roll core having an applied base layer is substituted in place of the disposable mold. An intermediate layer comprising a wax or other dissolvable material is applied over the roll base. The cover is then cast or wrapped over the intermediate compressive layer and roll base. Then the intermediate layer is dissolved away and the resulting gap is filled with an adhesive layer.